Internal combustion engines that are operated either constantly or at times with a lean air-fuel mixture produce nitrogen oxides NOx (mainly NO2 and NO), which require NOx-reducing measures. Exhaust-gas recirculation constitutes an engine-related measure aimed at reducing NOx raw emissions in the exhaust gas, and this is a process in which part of the exhaust gas of the internal combustion engine is recirculated into its combustion air, as a result of which the combustion temperatures are lowered and consequently the formation of NOx (NOx raw emission) is reduced. As a rule, however, exhaust-gas recirculation is not sufficient to comply with statutory NOx limit values, which is why there is an additional need for an active exhaust-gas after-treatment that lowers the NOx end emission by catalytically reducing NOx to form nitrogen N2. A known NOx exhaust-gas after-treatment involves the use of NOx-storage catalytic converters which, during lean operation (with λ>1), store nitrogen oxides in the form of nitrates and, at short intervals with a rich exhaust-gas atmosphere (λ<1), desorb the stored nitrogen oxides and reduce it to nitrogen N2 in the presence of the reductants present in the rich exhaust gas.
Another approach to convert nitrogen oxides in the exhaust gases of lean-burning internal combustion engines is the use of catalyst systems that operate according to the principle of selective catalytic reduction (SCR). These systems comprise at least one SCR catalytic converter that, in the presence of a reductant metered into the exhaust gas—normally ammonia NH3—selectively convert the nitrogen oxides of the exhaust gas into nitrogen and water. In this context, the ammonia can be metered into the exhaust-gas stream from an aqueous solution of ammonia or from a precursor compound, for instance, urea in the form of an aqueous solution or solid pellets, obtained through the modality of thermolysis and hydrolysis. A more recent approach for ammonia storage in a motor vehicle utilizes the NH3-storage materials that reversibly bind the ammonia as a function of the temperature. In this context, metal-ammine storage materials are well known such as, for example, MgCl2, CaCl2 and SrCl2, which store ammonia in the form of a complex compound so that it is then present, for example, as MgCl2(NH3)x, CaCl2(NH3)x or SrCl2(NH3)x. The ammonia can once again be released from these compounds by feeding in heat.
It likewise a known procedure to continuously check the proper functioning of exhaust-gas catalytic converters comprising SCR catalytic converters or NOx-storage catalytic converters by means of on-board diagnostics (OBD). Towards this end, the signal of an exhaust-gas sensor (NOx sensor) located downstream from the catalytic converter is normally employed for the appertaining exhaust-gas component in order to measure the concentration of this exhaust-gas component downstream from the catalytic converter. Furthermore, the concentration of the exhaust-gas component is determined upstream from the catalytic converter, that is to say, the raw emission of the engine. This can be done by measuring the concentration by means of another exhaust-gas sensor installed upstream from the catalytic converter. However, the raw emission is more often ascertained by means of modeling using stored characteristic maps that depict the concentration of the component as a function of the momentary operating point of the internal combustion engine. The efficiency η of the exhaust-gas catalytic converter in terms of the conversion of the component can then be obtained, for example, from the equation below, wherein c_end is the concentration (or contents) of the exhaust-gas component measured downstream from the catalytic converter and c_raw stands for the raw emission of the internal combustion engine regarding this component:
  η  =      1    -          c_end      c_raw      
The efficiency η can thus assume values from 0 to 1. An ideally functioning catalytic converter that brings about a complete catalytic conversion (c_end=0) thus exhibits an efficiency η of 1, whereas η=0 (c_end=c_raw) in the case of a completely inactive catalytic converter.
German patent application DE 10 2010 042 442 A1 describes an exhaust-gas system with an SCR catalytic converter as well as with a low-pressure exhaust-gas recirculation system by means of which an exhaust-gas stream is withdrawn downstream from a turbine of an exhaust-gas turbocharger (on the low-pressure side) and downstream from the SCR catalytic converter, while the combustion air of the internal combustion engine is fed in upstream from a compressor of the exhaust-gas turbocharger (on the low-pressure side). In the exhaust-gas recirculation line, there is a NOx sensor that measures the concentration of nitrogen oxides in order to regulate the internal combustion engine on the basis of the NOx concentration thus ascertained, especially the exhaust-gas recirculation (EGR) rate or the air-fuel ratio. The exhaust-gas recirculation line is also connected to the air line of the internal combustion engine via a bypass that opens up into the air line downstream from the compressor, in other words, on its high-pressure side. In order to determine an offset of the NOx sensor so that it can be calibrated, the bypass is opened, which brings about a reversal in the direction of flow in the exhaust-gas recirculation line, so that the NOx sensor is charged with fresh air. A diagnose of the SCR catalytic converter is not described here.